Observed streamflow and climate data are used to test the hypothesis that climate change is already affecting Rio Grande streamflow volume derived from snowmelt runoff in ways consistent with modelbased projections of 21st-Century streamflow. Annual and monthly changes in streamflow volume and surface climate variables on the Upper Rio Grande, near its headwaters in southern Colorado, are assessed for water years 1958-2015. Results indicate winter and spring season temperatures in the basin have increased significantly, April 1 snow water equivalent (SWE) has decreased by approximately 25%, and streamflow has declined slightly in the April-July snowmelt runoff season. Small increases in precipitation have reduced the impact of declining snowpack on trends in streamflow. Changes in the snowpack-runoff relationship are noticeable in hydrographs of mean monthly streamflow, but are most apparent in the changing ratios of precipitation (rain + snow, and SWE) to streamflow and in the declining fraction of runoff attributable to snowpack or winter precipitation. The observed changes provide observational confirmation for model projections of decreasing runoff attributable to snowpack, and demonstrate the decreasing utility of snowpack for predicting subsequent streamflow on a seasonal basis in the Upper Rio Grande Basin. (KEY TERMS: climate variability/change; runoff; snow hydrology; water supply.) Chavarria, Shaleene B. and David S. Gutzler, 2018. Observed Changes in Climate and Streamflow in the Upper Rio Grande Basin. Journal of the American Water Resources Association (JAWRA) 54(3): 644-659. https://doi.

Southwestern North America (SWNA) is projected to become drier in the twenty-first century as both precipitation (P) and evaporation (E) rates change with increasing greenhouse gas concentration. The authors diagnose the relative contributions of changes in P and E to the local surface moisture balance (P − E) in cold and warm halves of the year across SWNA. Trends in P − E vary spatially between the arid southern subregion (mostly northern Mexico) and the more temperate northern subregion (southwest United States), although both subregions exhibit a negative trend in P − E (trending toward more arid conditions) in CMIP5 projections for the twenty-first century. The P − E trend is biggest in the cold season, when much of the base flow to rivers in the southwest United States is generated. The downward trend in cold season P − E across SWNA is caused primarily by increasing E in the north and decreasing P in the south. Decreasing P is the primary contributor to modest warm season drying trends in both northern and southern subregions. Also, P accounts for most of the interannual variability in SWNA P − E and is strongly correlated with modes of oceanic natural variability during the cold season. SWNA aridification is therefore most readily distinguished from the region’s large natural climate variability in the cold season in the northern subregion, where the projected temperature-driven increase in E is greater than the projected decrease in P.

RESUMENSe describe la variabilidad interanual de ciclones tropicales cercanos a la costa en el Pacífico nororiental, utilizando para ello un conjunto de datos elaborado con reportes oceánicos y atmosféricos de EUA y México correspondientes al periodo . Los ciclones cercanos a la costa se enumeran de forma mensual, lo que permite distinguir la variabilidad interanual en distintas fases de la temporada de ciclones de mayo a noviembre. De acuerdo con estos datos, el número de ciclones tropicales que impactan la costa del Pacífico de mayo a julio (los primeros meses de la temporada de ciclones tropicales) en años correspondientes a La Niña, cuando las temperaturas marinas superficiales en el Pacífico ecuatorial son anormalmente frías, es mayor que en años correspondientes a El Niño. La diferencia en la cantidad de ciclones tropicales de inicio de temporada entre años de La Niña y El Niño fue especialmente notable a mediados del siglo XX, cuando se registró un incremento de las temperaturas ecuatoriales bajas, de acuerdo con un índice de la oscilación decenal del Pacífico. Los mapas combinados de años con conteos bajos y altos de ciclones tropicales cercanos a la costa muestran que las anomalías de la circulación atmosférica vinculadas con bajas temperaturas marinas superficiales en el Pacífico ecuatorial oriental, son consistentes con la trayectoria dominante de los ciclones tropicales hacia el noreste con dirección a la costa occidental de México. ABSTRACTThe interannual variability of near-coastal eastern North Pacific tropical cyclones is described using a data set of cyclone tracks constructed from U.S. and Mexican oceanic and atmospheric reports for the period 1951-2006. Near-coastal cyclone counts are enumerated monthly, allowing us to distinguish interannual variability during different phases of the May-November tropical cyclone season. In these data more tropical cyclones affect the Pacific coast in May-July, the early months of the tropical cyclone season, during La Niña years, when equatorial Pacific sea surface temperatures are anomalously cool, than during El Niño years. The difference in early season cyclone counts between La Niña and El Niño years was particularly Atmósfera 26(2), 149-162 (2013) 150 D. S. Gutzler et al. pronounced during the mid-twentieth century epoch when cool equatorial temperatures were enhanced as described by an index of the Pacific Decadal Oscillation. Composite maps from years with high and low near-coastal cyclone counts show that the atmospheric circulation anomalies associated with cool sea surface temperatures in the eastern equatorial Pacific are consistent with preferential steering of tropical cyclones northeastward toward the west coast of Mexico.

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The summertime sources and transport pathways of water vapor into the interior of southwestern North America are examined in a high‐resolution mesoscale model simulation. A complex multi‐stage transport pathway is associated with a pronounced diurnal cycle of deep convection and low‐level wind systems. Daytime thermal pumping of moisture from the Gulf of California towards the Sierra Madre Occidental (SMO) leads to widespread, deep convection. At night, land surface and atmospheric cooling generate drainage flows down both sides of the SMO. A nocturnal, low‐level jet forms above the western foothills and transports moisture north into the Sonoran Desert. Moist air draining to the east mixes with onshore flow from Gulf of Mexico and flows into the northern Chihuahuan Desert. The diurnal flow of moisture up and down the SMO and the formation of nocturnal, low‐level jets transport moisture into the core regions of the NA monsoon in NW Mexico.